A new study on heart medications adds credence to that old saying, "timing is everything."

Canadian scientists found that drugs known as ACE inhibitors - used to treat high blood pressure and heart failure - are far more effective when taken before going to sleep. In fact, when administered during wake time, they are no better than placebos, according to the findings published in the Journal of the American College of Cardiology.

This new study is part of a burgeoning field of research known as chronotherapy, in which medical treatments are timed to correspond with the body's natural 24-hour circadian rhythm.

The researchers at the University of Toronto and the University of Guelph already had good reason to believe the principles of chronotherapy may apply to these drugs. Previous research has shown that the cardiovascular system goes through a daily cycle, with blood pressure normally rising in the morning and dipping at night.

For their study, they gave a short-acting ACE inhibitor, called captopril (brand name Capoten), at different times of day to laboratory mice with high blood pressure. Heart tissues from the rodents were then examined as part of the analysis.

"We found that when we gave this drug at sleep time, the structure and function of the heart was significantly improved," said the study's lead author, Tami Martino of the University of Guelph. However, when the drug was given while the rodents were awake, it seemed to "have no effect at all" in terms of protecting the heart from the damage caused by high blood pressure, she added.

Although the study involved rodents, the researchers are confident the results also apply to people. "I wouldn't have any hesitation saying that, in most cases, you should give ACE inhibitors at night to patients with hypertension," said Michael Sole, the study's senior author who works at the Peter Munk Cardiac Centre at the University Health Network in Toronto.

He noted that many people diagnosed with hypertension suffer from high blood pressure during the day as well as the night. But elevated blood pressure at night is especially harmful because the heart is never given a chance to recuperate. "Sleep is not just for the brain; it is a very important period when our organs - including the heart - undergo repair," said Dr. Sole.

Pharmaceutical companies have introduced long-acting ACE inhibitors - such ramipril (Altace) and enalapril (Vasotec) - to control blood pressure for extended periods and keep the body on an even keel. But these medications can cause serious side effects - including kidney problems - in some patients.

Dr. Sole said it may be better to use a short-acting medication, like captopril, at night when the heart is in "repair mode" particularly in patients who find it hard to tolerate the side effects.

"If you time the drugs to match the body's natural physiology you can make them more effective," added Dr. Martino.

Unfortunately, the researchers noted, many patients who are prescribed long-acting ACE inhibitors take them in the morning, reducing the drug's potential benefit because the medication wears off during the night.

When hypertension is uncontrolled or poorly managed, it can lead to heart attacks, strokes and kidney damage.